The invention relates to an electrochromic light valve with an active electrochromic layer, i.e. one whose colour can change, and a passive electrochromic layer, i.e. one whose colour essentially remains unchanged, between which an electrolyte is arranged and each of which is connected to an associated electrode, at least one of which is transparent, the coloration of the active electrochromic layer being reversibly changeable by acceptance or donation of electrons via its associated electrode and of ions of a particular element or several particular elements via the electrolyte, the passive electrochromic layer simultaneously donating ions of the species concerned of the element or the elements to the electrolyte or accepting them from the electrolyte.
The invention also relates to a method of manufacturing an electrochromic light valve of this kind in which a passive electrochromic layer is produced which contains at least one of the elements which provide the ions of the species concerned.
The invention also relates to particularly advantageous uses of this electrochromic light valve.
Electrochromic light valves are known, for example, from FR 2 633 609 A1, GB 2 081 922 A1, WO 91/02282, EP 0 323 006 A2 and DE 40 10 285 A1.
Electrochromic light valves of the kind described above can operate according to one of the following two alternative mechanisms:
a) The first alternative mechanism consists of the active electrochromic layer becoming coloured by accepting electrons from the electrode on one of its layer sides and positive ions from the electrolyte adjacent to its other layer side, the resulting depletion of positive ions in the electrolyte being compensated by the passive electrochromic layer supplying positive ions of the same element or another element to the electrolyte and simultaneously donating electrons to the outside via its electrode. This mechanism operates for example in an electrochromic light valve whose active electrochromic layer essentially consists of tungsten trioxide (WO.sub.3) and in which the ions are lithium ions (Li.sup.+) for example. This mechanism can also function with negative ions instead of positive ions, in which case when becoming coloured the active electrochromic layer donates negative ions to the electrolyte and the passive electrochromic layer accepts negative ions of the same element from the electrolyte. PA1 b) The other alternative mechanism consists of the active electrochromic layer becoming coloured by donating electrons to the outside via its electrode and simultaneously accepting negative ions from the electrolyte via its interface with the electrolyte or donating positive ions to the electrolyte, the resulting ion depletion or ion excess in the electrolyte being compensated by the passive electrochromic layer donating ions of the species concerned of the same element or another element to the electrolyte and/or accepting them from the electrolyte and receiving electrons from outside in each case via its electrode. As an example of positive ions being donated when the active electrochromic layer is being coloured the literature quotes a system in which the active electrochromic layer essentially comprises iridium oxide (IrO.sub.x) whilst acceptance of negative ions by the active electrochromic layer when being coloured takes place for example in a system in which this layer essentially comprises nickel oxide (NiO.sub.x).
An electrochromic light valve of the kind described above, in which the active electrochromic layer can be reversibly coloured by accepting electrons from its electrode and positive ions of a particular element from the electrolyte, the passive electrochromic layer donating positive ions of the element to the electrolyte and/or accepting them from the electrolyte, is for example known from FR 2 633 609 A1 and the paper "Dip-coated CeO.sub.2 -TiO.sub.2 films as transparent counter-electrode for transmissive electrochromic devices" by P. Baudry, A. C. M. Rodrigues and M. A. Aegerter, which is published in Proc. Vth Int. Workshop on Glasses and Ceramics from Gels, Rio de Janeiro, 6-10 Aug. 1989, and also from GB 2 081 922 A in a non-transparent form and with two active electrochromic layers instead of one active and one passive electrochromic layer.
In the electrochromic light valve described in the first two above-mentioned publications, the transparent active electrochromic layer comprises a transition metal oxide, preferably tungsten trioxide (WO.sub.3), whilst the transparent passive electrochromic layer comprises a rare earth oxide, preferably cerium oxide (CeO.sub.u where u=2.3) or cerium-titanium oxide (CeO.sub.2 --TiO.sub.2). An organic polymer conductive to lithium ions (Li.sup.+) is preferably provided as the transparent electrolyte. In contrast, in the non-transparent electrochromic light valve according to GB 2 081 922 A the one active electrochromic layer comprises WO.sub.3 and the other also active electrochromic layer comprises Li.sub.x WO.sub.3.
The mode of operation of a transparent electrochromic light valve which, as stated in FR 2 633 609 A1, is used in particular as a sunscreen window for buildings and vehicles, is as follows:
The tungsten trioxide layer can be coloured by applying an electrical voltage between the two electrodes, i.e. by generating an electric field between the electrodes. This coloration takes place by light-absorbing colour centres being produced by electrons being injected into the tungsten trioxide layer from its associated electrode. In order to be able to inject a sufficiently large number of electrons into the tungsten trioxide layer and hence to produce a sufficiently large number of colour centres in it the charge injected into the tungsten trioxide layer by the electrons must be compensated. This is achieved by positive ions (e.g. H.sup.+, Li.sup.+ or Na.sup.+), in this case lithium ions, being simultaneously injected into the tungsten trioxide layer from the electrolyte. Similarly positive ions must be supplied Co the electrolyte from the passive electrochromic layer. Conversely, when the tungsten trioxide layer is de-coloured the passive electrochromic layer must be capable of accepting as many positive ions as need to be removed from the tungsten trioxide layer to de-colour it. This passive electrochromic layer comprising cerium oxide or cerium-titanium oxide therefore has the function of an ion acceptance and donation reservoir for the positive ions such as the lithium ions mentioned, for example.
In this case, as stated in FR 2 633 609 A1 for example, it is assumed that all the lithium can reversibly migrate to and fro between the active electrochromic layer comprising tungsten trioxide and the passive electrochromic layer comprising cerium oxide or cerium-titanium oxide, i.e. that all the lithium which is in the cerium oxide or the cerium-titanium oxide layer is available for the coloration of the tungsten trioxide layer.
It has now been found that in a transparent electrochromic light valve of this kind the degree or depth of coloration, i.e. the darkness or light absorption factor of the tungsten trioxide layer diminishes if the tungsten trioxide layer is subjected to a certain number of coloration cycles, e.g. 2,000 coloration cycles which are carried out at:the same coloration voltage and for the same coloration time. And the degree of coloration does in fact diminish to a considerable extent so that a transparent electrochromic light valve of this kind which is designed as a light-absorbing windscreen ceases to be usable in practice, because with such a relatively low available number of approx. 2,000 coloration cycles it is not sensible in practice to fit a light-absorbing windscreen of this kind into a vehicle or the like.
In particular the object of the invention is to make available an electrochromic light valve of the above-mentioned kind in which the achievable degree of coloration is essentially maintained, i.e. does not decline substantially, despite an increasing service and storage duration.